Mud motor

In today s operations a mud motor or a mud turbine are mostly used for directional drilling. Rotary drilling may be carried out between mud motor / turbine drilling i.e. the use of these is often restricted to a certain interval only. 

A mud motor (Fig. 3.17) is a positive displacement hydraulic motor, driven by the circulated drilling fluid. A continuous seal is formed between the body ( stator ) and the... 

Figure 3.14 Lengthwise and cross sectional cut through a mud motor...

An alternative type of downhole mud motor is the mud turbine, (multistage axial flow turbine) which directly drives the bit. The tool consists of an upper section containing the turbine blades and lower section with bearings. As mud is pumped through the upper section the blades are turned. Turbines are designed to rotate at higher speed than the displacement motor. The higher rotation speed requires diamond or composite bits. 

Diamond bits are being employed to a greater extent because of the advancements in mud motors. High rpm can destroy roller rock bit very quickly. On the other hand, diamond bits rotating at high rpm usually have longer life since there are no moving parts. 

Air (or Gas) Downhole Motors. Some positive displacement mud motors can be operated on unstable foam. In general, these mud motors must be low-torque, high-rotalional-speed motors. Such motors have found limited use in air and gas drilling operations where directional boreholes are required. Recently a downhole turbine motor has been developed specifically for air and gas drilling operations. This downhole pneumatic turbine motor is a high-torque, low-rotational-speed motor. 

A steering tool is normally used during drilling with a mud motor and is connected to the surface with an electric wireline. The sensing devices shown in Figure 4-232 are also used in most MWD mud pulse systems. The coordinates... 

After drilling the mud motor length with a given tool face angle and a given bent sub angle, what is the borehole deviation and orientation likely to be at the mud motor depth ... 

Note We will assume that the borehole axis is the same as the drill collar axis at the steering tool depth and also that the borehole axis is the same as the mud motor axis at the mud motor depth. 

Figure 4-234. Vector diagram showing the mud motor axis as weil as the steering tooi axis.

We are using a mud motor which rotates at 500 rpm and develops a useful power of 100 hp. Assuming a constant flowrate of 500 gal/min, no pressure loss in the bit nozzles and 80% motor efficiency, compute the total bottomhole assembly AP at 0.2- and 0.5-in. valve opening. 

A positive displacement mud motor is included in the downhole assembly between the bit and the MWD system. It develops a true power of 100 hp when the pulse nozzle is closed. What is the true power obtained when a 200-psi pulse is created The bit nozzle pressure loss will be neglected. Use... 

A typical MWD bottomhole assembly used in rotary drilling is as follows from bottom to top drill bit, stabilizer, resistivity, WOB torque, directional and telemetry system, neutron-density Pe. The typical distances are seen in Figure 4-296a. When a mud motor is inserted between the lower stabilizer and the drill bit, the distances are increased as shown in Figure 4-296b. 

To meet the challenges posed by horizontal drilling in particular, a system has been developed to make the measurements at or near the bit and transmit them to the mud telemetry section of the MWD bottomhole assembly. The resistivity at the bit tool is similar to the toroidal resistivity tool described in the section titled Resistivity Logs. As shown in Figure 4-297, the Anadrill geosteering package includes below the Power Pack mud motor ... 

Figure 4-296. Typical MWD bottomhole assemblies (a) rotary drilling (b) mud motor drilling.

Both rotary and mud motor systems use an electromagnetic wireline telemetry to relay the data from the near-bit sub to the mud telemetry sub. 

Downhole Mud Motor Optimization. Downhole mud motors in use today are practically all of the positive displacement type (sometimes called the moving cavity ). A description of these motors has been given in section 4.10. The theoretical characteristics curves of the motors are shown in Figure 4-315 in arbitrary units. 

Figure 4-315. Characteristics of new and worn out positive displacement mud motors. (Courtesy Anadrill.)...

Geosteering is usually done with a mud motor. A mud motor with bent sub allows changing of orientation and inclination without pulling the drillstring out. Steering is done by rotating it a small angle. 

In classical geosteering the sensors for inclination, azimuth, drilling parameters, and logging are located above the mud motor and the distances may be in the order of those shown in Figure 4-296 that is 30 ft or more above the drill bit. Although radial measurements can be performed to verify that the borehole is being drilled in the pay zone, it is often too late to make a correction and the borehole leaves the pay zone. 

The new geosteering system offers measurements at the bit (below the mud motor) of inclination, rpm, azimuthal gamma ray, azimuthal resistivity, and bit resistivity as seen in Figure 4-298. The signals are transmitted electromagnetically to the MWD sub located above the mud motor, then relayed to surface with the standard mud pressure transmission system. To summarize, the following is recorded Just above the drill bit ... 

Well No. 1 was drilled with inclination and azimuth data only. The sensors were located above the mud motor. Only a short section (63 m 207 ft) was drilled in the reservoir. 

No MWD or LWD equipment exist to date to log these wells. However, service companies are developing equipment for MWD purpose in 40-ft (12-m) curvature radius or 1.5°/ft (4.6°/m). The equipment consists of articulated mud motors and inclination and azimuth sensors. 

Medium radius and long radius wells are drilled with conventional oilfield tools. Both MWD and LWD are usable in these wells. Downhole motors are mostly used in medium radius wells to avoid fatigue of the BHA. Long radius wells have been drilled with both mud motors and rotary techniques. 

Mud motor Usually a positive displacement or turbine-type motor. 

If not, the bit, drill string and mud motors would fail more rapidly. Lubrication based on the coefficient of friction. Oil- and synthetic-based mud generally lubricate better than water-based mud (but the latter can be improved by the addition of lubricants). 

To provide hydraulic power to downhole mud motors if used... 

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